In order to distribute and store multimedia data, the multimedia data is transmitted over a communication channel. Multimedia data primarily refers to audio and visual data but may also include other types of data. The channel is often subject to noise and interference, as in the case of wireless channel, and to congestion, as in the case of a wired Internet, both resulting in loss of data during transmission.
Two methods can be used to combat data losses during transmission. Forward error correction (FEC) is a method of transforming the data message, represented by a sequence of symbols from a finite alphabet, by supplementing a parity data, another sequence of symbols, to ensure that if components of a codeword are altered below some designated threshold, the original data can usually be extracted intact. FEC therefore provides error resilience by increasing the amount of data to be sent. FEC does not require a return channel and is typically not adaptive to the current state of the channel. However, FEC does not guarantee that the data will arrive to the receiver without errors. A higher-level protocol implementing some form of repeat request for data that tolerates little errors is required for this to be addressed. Alternatively, in multimedia communications the delay requirements often dominate the error-free transmission requirements, making error-free transmission a lesser priority.
Basic automatic repeat request (ARQ) is an alternative approach to assist in robust data communications. ARQ operates by dividing the data into packets and appending a special error check sequence to each packet for error detection purpose. The data packets and error checks are communicated over a channel and the receiver decides whether a transmission error occurred by calculating the check sequence and comparing the calculated check sequence to the appended error check sequence. If a discrepancy is found, the error is declared and the receiver sends a request the transmitter using the return channel to resend the packet by sending a negative acknowledgement signal. If no discrepancy is found, the receiver sends a positive acknowledgement signal to the transmitter. To alert the transmitter of the error, ARQ requires a two-way communication channel to be present. Often, the return channel uses the same physical medium as the forward channel, effectively expanding the data size because of retransmissions and communication of control information. The difference between the FEC and ARQ is that ARQ is inherently channel adaptive, since only lost packets are retransmitted, while FEC typically adds overhead to all packets. Yet, ARQ may introduce significant delays due to roundtrip propagation time and processing time. The last condition significantly limits the application of ARQ to multimedia communications.